This invention relates generally to electrical sockets which mount an electrical part having a plurality of leads, such as an IC package, for burn-in testing or the like of the electrical part and more specifically to such sockets which readily mount and dismount an electrical part and which have an improved contact system for making electrical engagement with the leads of the electrical part.
It is conventional in manufacturing IC packages comprising a plastic sealed semiconductor integrated circuit chip (also hereinafter referred to as an IC chip) to subject the packages to a type of reliability test which is called a burn-in test prior to shipment in order to separate the packages into groups of satisfactory and unsatisfactory products. The burn-in test is for the purpose of placing an IC package that has passed a functional test in an oven and energizing the same for a given period of time at an elevated temperature, such as 120.degree. centigrade, under an electrical source which is approximately 20% higher than the rated value. Those IC packages which fail the burn-in test are discarded as unsatisfactory products and only those others which continue to function in a normal fashion are shipped out as satisfactory products.
An example of a prior art socket used for mounting IC packages for burn-in testing is shown in FIGS. 1 and 2. As seen in FIG. 1, the socket comprises a base 100 which serves as the main socket body to be fixed on a printed circuit substrate (not shown in the drawings) and a cover 102 hingedly mounted thereon. Hinge 104, having a coil spring 103, is disposed on one side of base 100 with bosses 106 of hinge 104 formed integrally on one side of the cover. Cover 102 is pivotable in a direction indicated by an arrow marked J about shaft 108.
In FIG. 2, a rotatable shaft 120 is journaled on the opposite side of cover 102 and mounts thereon a latch 122 for holding cover 102 in a closed state covering base 100. As hook 122a of latch 122 engages with a ledge 100a formed on the opposite side of base 100, cover 102 is closed on the upper surface of base 100. A lever 124 which is integral with latch 122 extends in the opposite direction and, as lever 124 is rotated in opposition to a coil spring 126, latch 122 is disengaged from ledge 100a of base 100, thereby making it possible for cover 102 to be opened.
The upper surface of base 100 is formed in such a way that an IC package 130 of the QFP type (Quad in line Flat Package) may be mounted at the IC package mounting position 105 at the center of the base. On the upper surface of base 100, four guides 132, 134, 136 and 138 extend upwardly in close proximity to the IC package mounting position. The two side surfaces inside a pair of guides 132 and 134 which face each other constitute inclined surfaces 132a, 134a, forming a tapered groove to guide IC package 130.
A large number of contacts 140 are provided in single rows on the upper surface of base 100 in such a fashion as to correspond to the row of leads of each side of IC package 130 receivable between two adjacent guides 132, 134; 134, 136; 136, 138 and 138, 132. A partition wall 141 is formed between each pair of adjacent contacts 140 in order to prevent the contacts from engaging each other.
Contacts 140 may be prepared by punching a thin plate of suitable material such as beryllium copper into a stationary part 140a which is fixed to base 100, a spring part 140b which extends upwardly in a curved direction from the top of stationary part 140a, a spring part 140c which extends straight in a horizontal direction from the top of curve spring part 140b, a contact part 140d which extends upwardly at a right angle from the tip of the straight line spring part and a socket terminal pin 140e which extends downwardly from the bottom of the stationary part 140a perpendicular to the bottom surface of the base and which protrudes therethrough. The upper edge of the contact part 140d functions as a contact edge 140f.
When IC package 130 is guided by grooves 132a and 134a and set at IC package mounting position 105, the tip of each lead 130a of IC package 130 is placed on a respective contact edge 140f of contacts 140. When the cover 102 is closed in this state, the force applying wall members 105a formed on four sides of the inner surface of the cover 102 engage the upper side of the tips of leads 130a of IC package 130, with a result that IC package 130 is pressed downwardly in opposition to the spring force of springs 140b and 140c of contacts 140.
As shown by a dashed line in FIG. 3 and the depressed position noted by a prime added to the numerals of the various movable parts, the straight line spring part 140c of contact 104 pivots downwardly about fulcrum F as the center with both contact edge 140f and lead 130a depressed by a prescribed distance of delta y and, along with this, the point of engagement of contact edge 140f shifts toward the base of the lead by a horizontal distance of delta x, thereby rubbing lead 130a. Due to the rubbing of contact edge 140f a film of oxide is peeled off the 10 surface of lead 130a, thereby enhancing the electrical connection between lead 130a and contact 140.
In sockets made in accordance to the prior art in which the contact elements are pivoted by force exerted through the leads of an electrical part when mounting the electrical part as described above, contact edges 140f engage leads 130a at a position restricted at all times to one side of an imaginary horizontal line HL passing through fulcrum F (center of revolution of contacts 140), i.e., above line HL in the example shown in FIG. 3. Because of this, the moving vector in the horizontal direction of each contact edge 140f is always in one direction (a direction toward the base side of the lead in the example shown in FIG. 3) with a concomitant large displacement delta x of the movement of contact edge 140f.
As contact edges 140f of contacts 140 move in sliding engagement with leads 130a, however, a shearing force is added to leads 130a which increases in proportion to the quantity delta x of the displacement of contact edges 140f.
In view of the fact that, in the case of the conventional socket, the quantity delta x of the displacement of contact edges 140f of contacts 140 is large, a large shearing force is applied to leads 130a, with concomitant danger of permanently deforming leads 130a or peeling off the plating on the surface of the leads.
FIG. 4 shows the construction of another prior art contact element in which contact edge 140f of contact 140' that engages lead 130a is adapted to shift to positions due to an extended curved spring part 140g which are always below horizontal line HL in which the fulcrum F (center of the revolution) of contact 140' lies, with a result that the movement vector in the horizontal direction of contact edge 140f is always in the direction toward the tip of the lead. As in the case of the FIG. 3 structure, the amount of displacement delta x of contact edge 140f of contact element 140', restricted to one direction is large, so that a similar disadvantage as described above is also characteristic of this type.
In sockets made according to the prior art as described above, if the amount of "depression", i.e., the amount of vertical displacement delta y, is set small in order to minimize the amount of the horizontal displacement delta x, there will be a concomitant reduction in the spring return force of contact element 140 or 140', resulting in another disadvantage in that it becomes more difficult to obtain sufficient compressive force for obtaining a suitable electrical connection.